1. Field of the Invention
The present invention relates to an optical pickup and an optical disc apparatus and is very appropriately adapted to, for example, an optical disc apparatus corresponding to an optical disc having a plurality of recording layers.
2. Description of the Related Art
In the related art, optical disc apparatuses recording information on optical discs such as a CD (Compact Disc), a DVD (Digital Versatile Disc), and a Blu-ray Disc (a registered trade mark, hereinafter, referred to as BD) and reading the information from the optical disc has been widely used.
In general, the optical disc apparatus collects a light beam through an objective lens and illuminates a track, which is formed on a recording layer of the optical disc in a spiral shape or a concentric shape, with the light beam.
At this time, the optical disc apparatus generates a focus error signal and a tracking error signal representing deviation amounts of the desired track, to which the focus of the light beam is aligned, and the focus of the light beam in the focus direction and the tracking direction.
Next, the optical disc apparatus allows the focus of the light beam to trace the desired track by performing the focusing control and the tracking control of the objective lens based on the focus error signal and the tracking error signal.
On the other hand, as a method of generating the tracking error signal, various methods are proposed. As one of the methods, a phase comparison method (sometimes, referred to as a DPD (Differential Phase Detection) method) is known (for example, refer to Japanese Unexamined Patent Application Publication No. 9-180212 (FIGS. 13A to 13C and FIGS. 14A to 14C)).
For example, FIGS. 1A and 1B diagrammatically illustrate configurations of a hologram plate 3 and a photodetector 4 assembled in an optical pickup 2 (not shown) of an optical disc apparatus 1 corresponding to the DPD method.
The optical pickup 2 is configured to illuminate the optical disc (not shown) with a predetermined light beam L, to divide a reflected light beam LR including the reflected light by the hologram plate 3, and to receive light so that each of the divided light beams is further divided when the light is received by the photodetector 4.
The hologram plate 3 is divided into two areas 3AD and 3BC in a radial (RAD) direction (the direction corresponding to the radial direction of the optical disc, that is, the horizontal direction in the figure) and is set so that the reflected light beam LR is diffracted in the opposite directions with respect to the radial direction by the holograms formed in the respective areas.
In the photodetector 4, a plurality of the light detecting areas 4A, 4D, 4B, and 4C are disposed to be adjacent to each other in the tangential (TAN) direction (the direction corresponding to the tangential direction of the track of the optical disc, that is, the vertical direction in the figure).
The light detecting areas 4A and 4D receive the spot TX corresponding to the area 3AD in a manner of division by two in the tangential direction to generate light detecting signals SA and SD. In addition, the light detecting areas 4B and 4C receive the spot TY corresponding to the area 3BC in a manner of division by two in the tangential direction to generate light detecting signals SB and SC.
Herein, as illustrated in FIG. 2B, it is known that ranges (hereinafter, referred to as phase changing ranges HA, HB, HC, and HD) where the light intensity is changed according to the tracking state of the light beam L are formed in the reflected light beam LR.
For this reason, the optical pickup 2 receives light so that independent light detecting areas 4A to 4D (FIG. 1) includes the corresponding phase changing ranges HA to HD. Therefore, the light detecting signals SA to SD appropriately representing the light intensities may be generated.
In addition, in FIGS. 2A to 2C, in an image of the reflected light beam LR, the division lines (indicated by solid lines) by the hologram plate 3 are repeatedly illustrated, and the division lines (indicated by broken lines) of the case where the reflected light beam LR is divided by the light detecting areas of the photodetector 4 are repeatedly illustrated. In addition, for the convenience of description, the areas of the case where the hologram plate 3 is divided by two division lines are referred to as divided areas 3A, 3B, 3C, and 3D.
The optical disc apparatus 1 generates phase signals φSA, φSB, φSC, and φSD indicating phases of the light detecting signals SA, SB, SC, and SD and, after that, generates the tracking error signal based on the DPD method according to the following Equation (1) by using a predetermined signal processing circuit or the like.[Equation 1]STE=(φSA−φSB)+(φSC−φSD)  (1)
After that, the optical disc apparatus 1 performs the tracking control based on the tracking error signal STE.